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Why do cargo airlines frequently choose passenger aircraft rather than aircraft designed specifically for cargo?
Cargo airlines are known to regularly use planes designed primarily for passenger travel.
Why don't they almost exclusively use aircraft designed specifically for cargo?

What do you mean with "specialized cargo aircraft"? Special aircraft like an Antonov An-225, or cargo aircraft like a Boeing 747 freighter variant?

@Bianfable I mean the first (aircraft designed specifically for cargo). I have edited the question for more clarity. Thanks for suggesting :)

Thanks for clarifying! The 747 was however primarily designed for cargo. That's why they have the upper deck with the flight deck above the nose, which can be fully opened.

@Bianfable Really?! Didn't know that. Do you have more details on how it became a ubiquitous passenger aircraft?

IIRC the 747 was designed to do both, to provide a large and economical passenger aircraft in the short term and a very effective freighter in the long term. The assumption of the designers was that subsonic passenger travel would not be around much longer. Supersonic air travel turned out to be just too damn expensive and impractical though, so the 747 stuck around in passenger as well as freight roles.

interestingly, the 747 is the result of Boeing's losing bid for what is now the Lockheed C-5 Galaxy.

This question is spiritually similar to "when Google took off, why did they opt to use an array of commodity PC hardware rather than high-performance computers?"

That question sounds awfully similar to this one: https://aviation.stackexchange.com/q/35779/878

This question is a bit confusing, @RiteshSingh  There are no aircraft "especially designed for cargo".  Like, "the" most specialized, best, from-the-ground-up, specific "cargo aircraft" is ........  the cargo variant of the major aircraft we all know.  There's nothing else, there are no "more cargo oriented" aircraft.

I guess my comment there is completely covered by Jan's answer, good one

Price, commonality, and size choice.
A very large proportion of an aircraft's cost is in design and certification. It's followed by the engines, the avionics, and the fuselage. Not much cost can be removed from an aircraft by removing the passenger-specific elements of design. That is done in freighter versions.
Freighter models like the 747-8F already realize most of the possible savings. Second-hand airliners are cheaper still, and they're not hard to convert. Rip out the interior, install cargo securing systems, and fly.
Commonality with airliners means airliner-derived freighters are easier to service and repair anywhere one needs to. And, as Dohn Joe rightly adds, get pilots for; cargo airlines are both a training ground for passenger pilots and a career path of its own.
Finally, there are more airliner models than freight-only aircraft models, so there's more choice of the right payload and range capability.
This reuse of passenger airliners for freight is a great example of efficiency. Passenger airlines tend to have very high utilization, putting lots of cycles on their airframes and lots of fuel through them, so they care for better fuel efficiency, availability, and passenger experience (or at least their expectations when booking, considering the 9-abreast 787) delivered by newer airframes.
Cargo airlines run looser schedules, because their clients don't care much for their flights leaving on time (air freight is usually offered in 2-5 days), which shifts the optimization focus from fuel efficiency towards capital costs. They are able to use up the rest of the airframe's life in a less-critical job, where the oversight is a bit more forgiving and delays due to lower availability are more acceptable.
The An-225 is special in that it occupies a class above every other freighter in capability. It does the jobs nothing else can, but, until recently, the only airframe built has been able to serve the whole world.

Another aspect of commonality is workforce. When airlines retire a certain type, e.g. the DC-10, then a cargo airline has plenty of pilots to choose from. In the case of the DC-10, apart from the US Air Force, FedEx is the larger operator. So, a cargo airline may save the cost of pilot training, since converted airliners come with pilots included.

Pure freight operations tend to fly significantly fewer cycles than passenger or combi aircraft.   And you don't care too much how ratty looking it is as long as it's reasonably reliable. This makes older used aircraft a lot more attractive from a business case perspective.  You have airlines that buy combination pax/freighter aircraft new, "combies" but these are still primarily pax aircraft that are able to carry paying freight rather than go partly empty on a route with a low load factor at certain times of the day.
The best source of used aircraft is pax aircraft being retired by airlines, before they're fully worn out, when they upgrade their fleets.  Many normal passenger airliners have freighter conversion kits available by Supplemental Type Certificate for large cargo doors.  So a old MD-11 that is at 70% of its airframe life and can be had really cheap, still has quite a lot of life left in it if it carries on as a pure freighter at maybe only 4 or 500 cycles per year.  With the modest cost to convert to a freighter (cargo door and interior) it makes for a really attractive business case.
If you paid the big bucks for a new pure freighter (as opposed to a combi, which is still primarily a passenger a/c), the capital costs would kill you; you have to fly the heck out of it to make enough cash flow to justify the capital costs and freight operations just don't have the frequency.

Why do cargo planes fly fewer cycles? Is it just the time spent loading them is more, or is there less demand for them to fly?

Nice answer. Is 4 or 500 a typo? Maybe 400 or 500?

@RichardErickson it's written as you'd read it: "four or five hundred", though not as in "four, or five hundred" - it really means "(four or five) hundred".

@user3067860 I suspect that is because they mostly fly longer routes.  4 vs 1 hour can matter a lot for passenger flights; but when most cargo is sent on N-day delivery, it's cheaper to just fly to a hub and have a truck carry it the rest of the way instead of loading onto another aircraft for a shorter hop.

Yes I was being conversational, so it should be read as if it was being spoken.

@DanNeely: Another factor is that most packages are much more tolerant of multi-hour layovers than people are.  If some flights containing Los-Angeles-bound passengers arrive in Chicago at 11:00am and some arrive at 7:00pm, it might be "possible" to fit both groups of passengers on one 8:00pm flight from Chicago to LA, but the passengers who arrived at 11:00am probably wouldn't like that arrangement.  Most packages, however, wouldn't care.

@osuka_ Thank you for following up. I'm not used to see it written "4 or 500".

There is no such thing as aircraft designed specifically for cargo.
The types like C-5 Galaxy, C-17 Globemaster, An-124 Ruslan, C-130 Hercules or A400M Atlas are not designed for just any cargo, but for carrying cargo to the front line in a war, with ability to land on a strip prepared in a day by a small team of battle engineers and option to para-drop cargo and troops. That's a lot of very special military requirements a commercial operator flying between properly equipped airports has absolutely no use for, and all of them reduce fuel efficiency and increase maintenance costs. That's why nobody ever ordered a Lockheed L-500, the civilian version of Galaxy.
For operating between reasonably equipped airports, the only difference between cargo and passengers is that passengers want windows, and some equipment in the cabin, and cargo needs bigger door to be efficiently loaded. So there are no specific cargo designs, just aircraft built or converted for cargo, without (most) windows, but with big door for easy loading.
The exceptions are:

The small fleet of An-124 Ruslans and the sole An-225 Mrija for the occasional exceptionally heavy or large cargo that does not fit in a 747F, which wouldn't exist if the aircraft were not cheaply available when the Soviet Union collapsed.
Special outsize cargo carriers A300-600ST, B747-400LCF and A330-743L and former B-377-SG, which are all just conversions of normal airliners with significantly wider fuselage for very bulky cargo.
On the smaller scale, there is some market for carrying cargo to remote or poorly developed areas. This is partly covered by ex-Soviet-military An-12s, but there is an occasional L-100 to be seen as well. It's still relatively small market.

... nobody ... ordered a Lockheed L-500 - And lucky they did not. That airplane went through rough teething. For example, I remember that for years cargo was limited to about 1/2 of its original spec until they could fix all the wing boxes. More than one airframe had an engine fall off in flight. No kidding. Read The C-5A Scandal. Military contracting, what a concept!

@radarbob Are there any still flying?

still flying. I believe they're up to "M" model. ... Not all bad!!  In 1974  an ICBM was launched from a C-5 It was mostly to send a message to the Soviets; That's one helluva military-unique requirement I'd say.

It looks like there will be new production of the civilian C-130, the LM-100J https://www.lockheedmartin.com/en-us/products/lm-100j.html

@trognanders, yes, there are and will be some civilian L-100 (C-130). For the lower capacity range there are still remote places where the very short take-off and landing and ability to operate without much ground support have their use. It is still small market compared to airport-to-airport cargo using more typical airliners.

Another factor is that airplanes (like almost anything else) become cheaper when you buy them in bulk. That means a massive capital investment up front. FedEx, for instance, was ready to buy 10 A380s at one point, but backed out due to production delays

FedEx Express, the express package delivery unit of U.S. shipping company FedEx, became the first customer to terminate an order for Airbus’ flagship plane when it scrapped an agreement to buy 10 A380-800F aircraft.
Instead, FedEx ordered 15 Boeing 777 Freighter aircraft and took options to purchase 15 more. The previous agreement with Airbus included an option for another 10 A380 planes, a FedEx spokesman said, but those options are now invalid.
FedEx said it expects to take delivery of four Boeing 777s in 2009, eight in 2010 and the remainder in 2011. At a list price of \$232.5 million to \$240 million each, the order will be worth at least \$3.48 billion to Boeing.

It's worth noting that FedEx Express flies their fleet almost constantly, which means they're getting a lot of value out of buying brand new. For most other airlines (like, say, National Airlines, which flies only on-demand air cargo) it makes better economic sense to rehab a passenger aircraft.

These days airlines often transport passengers by day and cargo by night. Seats are mounted on pallets, they can all be removed in about half an hour. Then the cargo goes in. The margins for passengers are shrinking but those for packages go up.
The Type of aircraft is called combi, one Example would be Boeing 747-400M, there are more current types.

Wow, I've never heard of this. Do you have a picture or source for this?

I've flown overnight flights before and I've certainly felt like I was packed in like cargo, this explains so much!! :D

I wouldn't say "often", and very few of the aircraft were designed for quick changeover -- the only ones I've found were the 727-100 and the 737-200.


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Why was Pan Am Flight 103 flying over Lockerbie?
Pan Am Flight 103 was a flight from Frankfurt to Detroit with stops in London and New York.
Why after taking off in London did it fly all the way to Lockerbie instead of going directly to New York? Take a look at these pictures.

On your map projection, is Greenland really, really big?

I remember watching an episode on this aired on Air Crash Investigation/Mayday on NatGeo and recall this 747's glorious title: the Clipper Maid of the Seas.

The direct route you show is actually only a straight line on your map projection. The surface of the Earth is curved and the straight line between London Heathrow and New York JFK looks like this (courtesy of greatcirclemapper.net):

That still does not quite get you over Scotland, but the actual flight path over the Atlantic typically uses a North Atlantic Track. These are routes, which are redefined twice a day to make optimal use of the prevailing enroute winds. Here is one example for such tracks:

(image source)
The entry points to these routes can be quite high north, therefore resulting in an optimum route over Scotland.

This whole topic is widely discussed. The path was, I believe, called the "Daventry Departure" from Heathrow, and was commonly used.

Yes, the Daventry Departure goes north from Heathrow, but only about half the way to Lockerbie, so the SID is not the only explanation here.

"That still does not quite get you over Scotland" It's quite a long way off! Lockerbie is about where the bottom of the "n" is in "United" and it's only about 10km inside Scotland. The great circle is about 200km from Lockerbie.

Because the closest distance between two point on the Earth’s surface is a curve (we are NOT having a flat earth debate here and further posts, comments or answers on that subject will be promptly deleted).
Said curve will run out over England, Greenland, then Canada and the United States.
Airliners, because of the nature of jet routes, departure, and arrival procedures often do not fly purely direct routes to their destinations.
Route selection can also be done as a compromise between to shortest route and headwinds aloft.  A more circuitous route to a destination may be a little longer in mileage than a more direct route, but might save gas if headwinds are lighter on the former course.

That's need not be the primary reason why to fly this North.

Sure it is.  You ever seen high altitude enroute charts for that region across the Atlantic?

Yes. But you don't mention any of these. You mention only curvature and closest distance, but that does not explain why to fly that much North of it.

See item #3 on my list.  I DO answer it.

up-voted to counteract the flat earther.

If the earth were a perfect sphere (i.e. a ball, constant distance to the centre everywhere on the surface), the (closest) distance between any two points would be a straight line, it's just not very good for planes (they do well up in the air, not through the earth's interior).

@JJJ What do you mean by "straight line"? Even if the earth was a perfect sphere, shortest paths would still be circular arcs. For example, the shortest path between two points at the same latitude, except the equator, would curve towards the pole. The shortest distance is only a straight line if you tunnel through the ground.

@DavidRicherby exactly, and that's why others have mentioned great circle distance.

@JJJ OK, so your comment is just being obtuse. I don't think it's at all helpful: it confused me and I know what the shortest path is, and I can only assume it's more confusing to anyone who doesn't.

@DavidRicherby The term straight line is sometimes used for great circle lines because these Geodesics are straight on the curved surface (2D). The straight line you describe (tunnel) is the 3D straight line (in flat, i.e. not curved, space).

@DavidRicherby and that's why I'm pointing out that it's easier to use great circle distance, especially when explaining the technicalities required in this question.

As shown in the map in Bianfable's answer, the great circle route from London to New York does not go over Greenland, contrary to what this answer claims.

@HenningMakholm Indeed -- you need to fly London to somewhere much farther west. Heathrow-Denver just grazes the southern tip of Greenland, for example.

Re "said curve will run out over England, Greenland, then Canada and the United States": Lockerbie is in Scotland (which is not part of England). See: https://en.wikipedia.org/wiki/Countries_of_the_United_Kingdom

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Gas Turbine Engine Starting Sequence

How to explain the figure above?

Welcome to aviation.SE. What is exactly that you don't understand of the figure you posted?

Related: How are turbine engines started?

I'll use the start sequence of a JT-9D on a 747-200 as an example. It's typical, and since there was no FADEC and since it took two people having to manually do the individual items that will better explain things.
The f.e. turns on the engine ignition switches to energize the igniter that will ignite the fuel when introduced.
The figure doesn't show the next step, that of starting the engine rotating. The f.e. did that by opening the start valve which directed pneumatic air into the starter. Starter operation was confirmed by the f.e. observing N2 rotation—the figure calls that the compressor. Some gas turbines use an electrical starter rather than a pneumatic starter.
At 30% N2 rotation, the captain opens the fuel valve and light-off occurs. The captain and the f.o. monitor the EGT to confirm light-off and to avoid an overtemp—the figure calls that the Peak Starting EGT.
At 50% N2 rotation, the engine can accelerate on its own. The f.e. turns off the igniter and closes the pneumatic vale supplying air to the starter.
The engine continues to accelerate on it's own to idle rpm.
All sorts of things can go wrong, and you practiced those things in the sim, and occasionally you had to contend with them in real life.

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Hydro-pneumatic fuel controls and hydro-mechanical fuel controls
What are the advantages and disadvantages of the hydro-pneumatic controls against the hydro-mechanical controls?

They are actually the same thing. Hydro mechanical fuel controls often include the use of pneumatic pressure to regulate the fuel flow. Here’s an example of a simple one: http://blog.covingtonaircraft.com/wp-content/uploads/2015/08/fuel-control-unit-pt6a.gif
The mechanical part is the input of rpm by a shaft connected to the compressor spool. The pneumatic part is the P3 (compressor exit pressure) input.


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What is the fuel source for the APU on a B737?
Where does the fuel come from that powers the APU on a 737? My understanding is that it is from the left front main fuel tank, but I've also heard, hearsay, it can have it's own source. I know that the methods for getting the fuel to the APU can vary: GPU, gravity feed, main tank switch. But where does the fuel come from?
Thank you

Thank you Ralph J, that helps solve a discussion I've been having with another pilot

It comes from the left side of the fuel manifold, which means it is most often fed from the #1 main tank, typically by turning on either or both boost pumps in that tank. However, it can be fed from the center tank (using the left center tank boost pump) or from the #2 main tank using a boost pump there and opening the crossfeed valve.
If no boost pumps are running, it suction (not gravity) feeds from the #1 main tank. This would be the case when starting the APU with only battery power and no external AC power. It isn't preferrd to do this for longer than necessary, though, because it puts more wear on the fuel control unit with the suction feed than when the fuel is being supplied from a boost pump.
There is no separate fuel tank dedicated to the APU. Gravity feed wouldn't work because the APU is higher than the main tanks (in the wings) and the center tank (below the cabin floor).  (The exception to that would be if the aircraft is inverted, although that situation is considered undesirable for several reasons unrelated to providing fuel to the APU.)

That last parenthetical feels like the understatement of the day.

Additionaly to what was answered by Ralph J, Boeing offers an additional DC fuel boost pump for APU, that runs whenever APU is running and pressure from the AC pumps from the left manifold is low (i.e. the pumps are off).

Nice find - I wasn't aware of that option, since it isn't installed on our fleet.  And, welcome to Av.SE!


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Can a delta wing fighter engage an arrestor net without its main landing gear?
A delta wing fighter aircraft with underwing drop tanks while landing loses its main landing gear, but the nose wheel is extended. Is it an option for the pilot to engage the barrier net of type EP-15 or MENA-30 at the end of the runway, or will the pilot eject?
Which is the safest and why?

What is the net for in this scenario? I would think all of that metal scraping along the runway would provide more than enough braking power.

Why is the delta wing a factor here?

In August 2009, a Saab Gripen pilot forgot to extend the landing gear, and walked away from the landing unharmed. And this was without any preparation of any kind.
There is no reason to eject from a perfectly good airplane just because you can't extend the landing gear. Belly landings are a pretty "routine emergency" (if that term makes sense).
The metal scraping across the runway will probably generate as much if not more friction than the tires would, so the risk of a runway overshoot is rather negligible, and there is no need to use any arresting gear at the end of the runway.
What could make sense, though, is to use arresting gear at the beginning of the runway, if the aircraft is equipped with an arresting hook and the runway is equipped with arresting wires.
It might make sense to drop the drop tanks if they are full.
The delta wing does not play a role here, this applies more or less to all kinds of wing planforms.

Well, I am told Delta wings are for unstable aircraft hence belly landing is not recommended. Here actaully Iam looking for information on whether or not to engage the Arrester Barrier Net of EP15 type if aircraft is not rolling on main landing gears.

The presence on nose landing gear provides an angle of attack for the wing to generate lift and hence the aircraft doesnot lose speed in the absence of  brakes.


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Are there take-over requests from autopilots?
I am thinking of the driver take-over request in automated driving, when the system limits are reached and the back-up driver has to intervene / drive manually. Is there something similar in aviation, e.g. autopilot is engaged and has reached its system limits? Maybe you can give me some examples.
Edit: The reason why I am asking is, that there is a lot of concern about take-over situations in the automobile industry because cars are getting more and more automated. Since automation has a longer tradition in aviation than in automobile industry I assume there is a thing or two to learn from you guys. So basically I want to know what take-over situations there are and what is done to make take-over easy for the pilot (airplane or helicopter pilot).

The most important thing that makes take-over easy is something that can't be duplicated in cars: unexpected situations in airplanes almost never require an instantaneous response.  You can afford to take a few seconds after the alarm sounds to establish situational awareness.

I assume in your question the autopilot is engaged. When the auto-flight system reaches the limits of its capabilities it simply disconnects. On Boeing aircraft you get a
> AUTOPILOT DISC

Warning Message on the EICAS and the siren sounds. Now it is all manual flying for the pilots.
An example would be turbulence exceeding the auto-flight system capabilities to maintain the intended trajectory.
After the question was clarified:
The big difference between flying and driving are

system redundancy - in an aircraft, everything important is available two times, the very important stuff even three times. If there is a technical problem system performance normally degrades accompanied with appropriate annunciations to the pilots.
the environment you are operating in - in the air there is no road to follow or you fall off a cliff. There are mountains, yes, but in cruise flight (similar to driving on the highway) where your vigilance is lower the mountains are below you. In the end, the airway is an imaginary thing. An imaginary line drawn between two waypoints. You won't crash (immediately) if you deviate from it. Even your surrounding traffic is miles away laterally. When the margin for error gets smaller pilots are way more vigilant. During departure/arrival/landing, even when flying fully automated, the concentration is fully on the aircraft. So deviations from the intended trajectory are noticed early and can be acted upon quickly. This even goes so far that with most commercial operators there is a sterile cockpit rule below 10,000 ft where there is no communication in the cockpit which is not relevant to the current situation.
the system sophistication - the feat car manufacturers are trying to accomplish is way bigger than an auto flight system for an airliner. The margins for error are way smaller, yet the precision needs to be way higher. In aircraft the autopilot is basically a glorified feedback loop. And even if the autopilot system fails spontaneously due to technical problems the aircraft initially keeps flying without any intervention and only slowly deviates from its current trajectory as aircraft are designed to be aerodynamically stable.
training of the operator (pilot/driver) - commercial pilots train for abnormal situations in full flight simulators every six months. Drivers, well, don't. Their last training was most probably when making their drivers license many years ago. Another thing is vigilance and planning ahead. Pilots are trained to anticipate what is coming next. You try to get a mental picture of the traffic around you and where conflicts may arise. And as your aircraft is moving typically pretty fast and there are no brakes you can slam you are flying in a rather defensive way.

And I even have a real world example where the auto flight system performed flawlessly yet human intervention was required:

What you see here is a blurry photo of the incorrect turn construction of a departure. The departure trajectory is supposed to be a left turn to the next waypoint (white star). The flight management system constructed a right 270° turn (magenta line) to match all the constraints of the waypoints (flyover, etc) where a 70° left turn would have been the correct behavior. The aircraft was flown manually to adhere to the departure procedure.
In this case no system degradation or warnings were present. You can see that the Flight Director (FD) on the PFD on the right commands a right turn (vertical magenta line). The autopilot simply follows FD commands and would therefore fly this (wrong) turn.

Thank you for your reply! Yes that is what I mean, I edited my original post. Follow-up question: Is there some support for the pilot to get back in the loop? I imagine the pilot has his attention elsewhere and might be surprised by the take-over request.

@Antonie The pilot should not have his attention elsewhere. And the normal instruments should be sufficient to get your bearings in such an event.

@ratchetfreak, it's easy to say "should not", but humans are unable to maintain passive vigilance for hours on end.

@Antonie there might be some startle effect in the beginning. Lets assume you have no idea what the proper trajectory would be. First advantage: the situation is seldom really time critical. And then you get some clues. A general good idea is to keep the blue up and the brown down on your artificial horizon. Assuming the auto thrust system isn't working as well - just dial in a proven pitch & power value. For cruise this most probably would be something around pitch +2.5° and power around 90% N1. Now you won't fall out of the sky. Time to rebuild your desired level of automation.

@Antonie the AP disconnect horn is very, very (very) audible and recognizable. Also it rings every time you manually disconnect the AP, so your brain quickly connects this sound to "manual-flying-mode". I only have around 30 hours as PIC and the autopilot disconnected on me once because of some sudden wind that made the PA-28 roll a little bit too much. Despite of my little experience, my hands grabbed the yoke and I leveled the plane as soon as the loud horn started ringing without me having to even think about it.

Adding to this, part of your system redundancy point: even the pilots are redundant. In airplanes with a flight engineer position, it's even double redundant (three qualified people minimum in the cockpit normally, though typically only two who are qualified to actually fly the plane). Either pilot can call another pilot's attention to an issue, and of the two pilots up front, any one can take over immediately if necessary. Try that in a typical car...

@aCVn that's not true for most GA aircrafts though.

commercial pilots train for abnormal situations in full flight simulators every six months. Drivers, well, don't. Well, drivers train for abnormal situations everyday, because they happen every day. The rain, the fog, the other vehicles drifting across lanes, pulling out, the coned off lane, the stalled vehicle... Abnormal situations are just normal everyday occurrences on the road.  : )

@GrimmTheOpiner that training comes from the "school of hard knocks" and often teaches exceedingly bad habits and responses (ever watch someone try to merge onto a highway at 40 MPH when traffic is doing 65+ MPH?). Pilot training is often written in blood (this caused a fatal accident in the past so we'll train pilots not to do it in the future), and is drilled into pilots on a regular basis after very thorough analysis of what the proper response should be. I get your point, unfortunately, it's not a good one.

in an aircraft, everything important is available two times not Boeing 787 MAX...

Thank you all for your contributions! To summarize: There is no assistance to help the pilot to get back in the loop immediately after a disconnect. The reflex triggered by the alarm, the fact that the pilot should have his attention on everything anyway (i.e. be not out of the loop), the fact that it is usually not very time critical and the reasons Chris stated above are enough.

There are many possible reasons for the autopilot to disconnect. The details will depend on the aircraft, but here are the criteria for a Boeing 737 (source: 737 NG FCOMv2 4.20.2 Automatic Flight - System Description):

Autopilot Disengagement
The A/P automatically disengages when any of the following occurs:

pushing either A/P disengage switch
pushing either Takeoff/Go-around (TO/GA) switch with a single A/P
  engaged in CWS or CMD;

below 2000 feet RA or,
with flaps not up or,
G/S engaged

[Option - A/P auto-disengages for TO/GA above 2000 feet RA]
  [Option - Honeywell -708 FCC and on]
  pushing either Takeoff/Go-around (TO/GA) switch with a single A/P
  engaged in CWS or CMD above 2000 feet RA with flaps not up or G/S
  engaged.
[Option - Fail-Operational Autoland]
  pushing either TO/GA switch after touchdown with both A/Ps engaged in
  CMD (except with LAND 3 or LAND 2 annunciated)
pushing either TO/GA switch after touchdown with both A/Ps engaged in
  CMD
pushing an illuminated A/P ENGAGE switch
pushing the A/P DISENGAGE bar down
[Option - Fail-Operational Autoland]  activating either pilot’s control wheel trim switch (except with LAND 3
  or LAND 2 annunciated)
activating either pilot’s control wheel trim switch
moving the STAB TRIM AUTOPILOT cutout switch to CUTOUT
either left or right IRS system failure or FAULT light illuminated
loss of electrical power or a sensor input which prevents proper operation
  of the engaged A/P and mode
loss of respective hydraulic system pressure.

As Chris already said in his answer, the A/P disengagement will trigger a warning horn and also illuminate the Autopilot (A/P) Disengage Light (number 1 in the following image from the FCOM):

The pilot flying will immediately take over manual control of the yoke and fly the aircraft. This is typically not as time critical as in a car, where a small deviation from your lane on the highway could result in a collision within seconds. The only phase of flight where A/P disengagement is critical is during an Autoland, where the pilot flying is already fully focused on monitoring the A/P and is ready to take over at any moment.
Re-engagement of the autopilot is possible whenever the autopilot engagement criteria are fulfilled:

Each A/P can be engaged by pushing a separate CMD or CWS engage switch. A/P
  engagement in CMD or CWS is inhibited unless both of the following
  pilot–controlled conditions are met:

no force is being applied to the control wheel
the STAB TRIM AUTOPILOT cutout switch is at NORMAL.

There is also a system on some aircraft requiring some input from the flight crew after a set time, similar to car systems complaining when you drive with cruise control and lane assistant without touching the steering wheel. For example, on the Boeing 777 (source: 777 FCOMv2 15.20.11 Warning Systems - System Description):

Crew Alertness Monitor
The FMC continuously monitors switch action on the MCP, EFIS control panel, display select panel, CDUs and radio transmitter microphone switches. When a predefined time elapses after the last switch action was detected, the EICAS alert message PILOT RESPONSE is displayed.


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Lift and Drag Coefficients using load cell data
I have a MAV and I need to find out the lift and drag coefficients for the wing and tail. I am not available with a wind tunnel so I will be using a load cell along with a separate wind source. Now I can measure forces on the MAV, airspeed at the wing and tail surfaces.
If I just use the angle of attack at the centre of mass then I can find the corresponding lift and drag coefficients (since there are two unknowns and two equations for forces along x and z axis). Now if I want to find separate lift and drag coefficients for wing and tail, for the corresponding angle of attacks, how am I going to do that? Once again I have force on the MAV in the x and z axis, angle of attacks for wing and tail, air speed, two equations of forces but four unknowns.
Please guide me resolve this issue.

You'd need to test the wing separately from the tail, one might think.  Doing this outside a wind tunnel seems like a lot of opportunity to introduce errors into your numbers... wind tunnels are used instead of load cells for some pretty good reasons, even though they're expensive to build & to maintain.  Also, what is a MAV? Only thing I come up with is Mark Watney's "Mars Ascent Vehicle" but I don't think that's what you're testing.

Thanks for your reply. I don't have access to a wind tunnel and a load cell is the next best thing available to me. MAV stands for micro aerial vehicle.

I guess your design is a conventional one.
One approach could be integrating two load cells in between the tail and wing sections.
You can measure drag force and moment. And the moment may help you estimate the tail lift (with some serious assumptions though).
If space is a concern, you can try building a scaled model (2:1?) to test a larger model. The resulting forces and moments must be corrected for Reynolds effects.


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Freezing cold cabin housing just under a window
I flew on a Boeing 737 plane yesterday and was sitting next to 2 windows. When I tried to rest my arm on the inner housing of the cabin during the flight, I noticed that the inner cabin housing just below one of the two windows was very cold. It was almost like touching a surface in your freezer. I was under the impression that it is not a common thing and that it could only result from outside air being in contact with the inner cabin housing which I believe is not supposed to happen.
I spoke to a flight attendant about it and he came check the window, but I feel like his feeling all in all was that I was worried for nothing, and I’m not sure he actually shared the information with anyone - and he did not thank me in anyway for reporting a potential issue.
I’m curious of anyone views on this issue. My uneducated guess was that it was maybe not a critical issue now, but a defect that should be known by maintenance teams as it could be tied to a structural issue of the outside window.

Related: Is freezing of windows normal in airlines?

It's not a structural problem, and nothing to worry about. The cabin of an airliner is a pressure vessel, the pressure outside of the airplane is far lower,  if there was a leak air would be going out, not in. The air outside is very cold, which makes the skin of the airplane cold, there's insulation between the skin and the cabin fixtures but if that comes off a bit you get cold spots.
By all means tell the cabin crew so they can log it for the next time the airplane's in for maintenance, it's important for passenger comfort.

Thank you for the very informative answer

I had a similar situation. It was during a flight from KTW to MIR. There was actually more or less 15 cm diameter and a couple of mm thick ice layer under my window inside the cabin.
While I see that other answers seem to suggest ignoring it, please read until the end of the story.
I'm a frequent flyer (used to have Platinum Topbonus membership when AirBerlin was still alive) and I've never encountered ice inside the cabin and a stream of really cold air right under the window before. I've decided to call a flight attendant.
As you could expect she told me initially with a very nice smile that ice on the window is completely normal. Same reaction as in other answers here - no offense. But when I finger-pointed the ice and said that I'd be fine with some ice on the window but not quite a chunk of ice under the window... her smile disappeared instantly - "Thank you sir, I will tell the pilots".
Less than 30 seconds later the captain came to check it. After a while of looking and touching he told me exactly this - "Thank you for reporting it. This is a problem for sure but don't worry - we definitely don't have a big breach in the fuselage and we are about to start the approach in 5 minutes so diverting to a different airport is not necessary. I will report it after landing - it has to be fixed before the next flight. You can change your seat if you don't feel comfortable here."
Don't be afraid to ask stupid question or report anything you find unusual during the flight. It's about the safety of you and other people on board. Just be polite, patient, cooperative and don't call them every 5 minutes :)

The air-conditioning packs are located under the floor, between the wings ahead of the landing-gear bays.
There is a duct which carries cold air to the crown of the fuselage and it is then distributed fore and aft. If I remember correctly, its usually located where the leading edge of the wing joins the fuselage. If you were seated around this area this duct could be the reason for the cold surface. There is usually insulation around the duct as mentioned by GdD and this could have been dislodged. Also the ducts are assembled using clamps and one could have come loose. Both these conditions could result in cold area leaking out and cooling the interior panels.


text:
Pilot's license for ENFP?
I'm an ENFP and really interested in getting my private pilot's license. I was wondering - are there ay other ENFP pilots out there? If so, do you like being a pilot? I'm afraid I might get bored with it, as things get rather routine for me fairly quickly. Is it a job that involves people/creativity would you say? Thanks! =)

"Flying is hours and hours of boredom sprinkled with a few seconds of sheer terror."

Enlighten this nearly 80 year old. What is ENFP?

@Terry Personality type. https://www.16personalities.com/enfp-careers

It’s a personality type Terry.  https://www.16personalities.com/enfp-personality

@ymb1 So personality disorders correlate with INTP? I think I have one of those laying around.)

I highly encourage everyone to read this page: http://www.code7700.com/psychology_101.htm#pilotpersonalitytypes .

I also encourage everyone, in the nicest way I can, to appreciate that not all people who fit into the same category perform in the same way or have the same interests...

Welcome to Stack Exchange! It looks like your question has been closed because this site isn't a good fit for it. Keep in mind that we don't think it's a bad question, but this site is designed to handle factual questions that have a single "right answer".

Being an ISTJ, I can't completely relate to your thought processes. I will say that flying taught me many things about my thought processes that probably nothing else could. Many aspects of flying are repetitive actions, such as check lists and cockpit flow procedure (S). On the other hand, understanding and interpreting weather is quite fluid (N?). I can say that I have never been bored while flying in the left or right front seats. I suspect that an ENFP, with the will to do it, could probably take an intro flight and progress to being a competent Private Pilot without ever getting bored. But professional flying is a different matter all together. Remember, with respect to boredom and aviation, complacency kills.
Hopefully an ENFP will chime-in.

I see no reason that an ENFP, or any other kind of personality cannot become a good pilot.  I encourage you to try.  Take a discovery flight at a local airport and see what you think.


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Why are the colors of the MiG-15's attitude indicator inverted?
Why are the colors of the MiG-15's attitude indicator inverted (blue on the ground side, brown on the sky side)?

Source: http://www.warbirdalley.com/mig17.htm

https://youtu.be/XZ_Ju1By7_U

It is in rolling on the taxi way and then flying level

Interesting.  Do you know that that is how they were originally instrumented?

No I don't know , I believe this is restaurateur fault with electrical engineering for that instrument.

There are many photos of the negative tilted  Attitude indicator

In Soviet Russia, Attitude Indicator inverts YOU!

To confuse Clint Eastwood. (...)

Apparently not related: How is a confusion possible between Western and Russian Attitude Indicators?

Wait a minute, I understand perfectly. In America, you pull up the plane to point it at the sky. In Soviet Russia, sky pulls up onto plane!

that's both hilarious and awesome, @TerranSwett !

That attitude gyro reacts in the opposite manner to pitch movement (pitch down = horizon line rolls DOWN) compared to the "conventional" manner (pitch down, horizon line rolls UP). This is sufficiently confusing that on similar gyros of American manufacture, the words CLIMB and DIVE are printed on the "ball" inside the gyro case to remind the pilot what the instrument is telling him or her.
The "opposite" reading gyro horizons were commonly used in American fighter aircraft because their construction made them more resistant to tumbling than were the "conventional" reading instruments, which were used in civilian aircraft.
After WWII, those opposite-reading gyros wound up on the surplus market and a number of them were installed in civilian aircraft, including the Beech Bonanza that was carrying Buddy Holly, Ritchie Valens and the Big Bopper on the night of their crash. The pilot had trained in a plane with a conventionally-reading instrument in its panel and the accident report claimed that he was confused by that opposite-reading gyro in the dark, thinking he was trimming for an 800FPM climb when what he was really doing was trimming for a descent- and minutes after takeoff, the plane flew smoothly into the ground and everyone aboard was killed.
I have an example of both types of gyro horizon in my collection, and even when they are not spinning, simply tilting their cases in unison demonstrates the difference: one rolls up, the other rolls down when you pitch them both down.

My God, that is confusing / amazing.  That's like Nintendo famously have the Y axis the wrong way; so anyone who's a pilot or has even just played any plane game gets stunningly confused - you push the stick forwards in Splatoon and you .. point upwards.  (Fortunately you can change it in the settings.)  Conversely kids who got a Switch and have never, you know, flown a plane have their minds wired the wrong way now - and hopefully they will never become pilots  :O

@Fattie Apparently it was Quake that's ultimately responsible for screwing this one up for everyone.

fascinating @MasonWheeler , thanks for that !

@Fattie I'll note that for an FPS, up=up, down=down makes sense.  As soon as you're in a vehicle though, that had better switch so that up=push nose and down=pull nose

it's definitely an issue, @Baldrickk !  Example my kids use "Nintendo style" when blasting around in Splatoon and it gives me a migraine to even watch their fingers while! :)   Humorously, from time to time I'll forget to swap it to what I consider "normal", if they've been using their machine, when I play   :)

This is interesting! The only lead I was able to find (so far) is this discussion thread, with a comment:

The colour becomes logical, if you see it moving.
  Climbing the plane which shows the sky in blue on the artificial horizon.
  Ground being brown/earth, if you see this, the nose of the plane is pointing downwards

And the explanation for arranging the colours in this way is, plausibly (same discussion):

It makes the instrument less complex. The way it is built means you do not have to invert the pitch axis movement of the gyro, which is the case for "conventional" artificial horizons.

If you want to build the simpliest possible attitude indicator, you take a gimballed gyro and attach a sphere with horizon line and some kind of metering lineage on it. This sphere will maintain its position in relation to earths surface. With roll it is self explanatory, it will indicate the tilt of the horizon as it will appear if you look outside. Indicating pitch is not as simple. As you pitch up, the horizon line will move up on this simple AI. Now that's not very intuitive. Pitching down, the line goes down. Again, not that intuitive!

Note how the pilot would see the bottom of the sphere: more blue (source; edited)
So, to keep the mechanism simple, they fixed this with colours: bottom part is blue, because when you pitch up, the bottom part will become more visible, thus indicating you are heading towards the blue sky. Pitch down, and the indicator will show more of its top, which is painted brown, indicating you are headed towards dirt.
You can see the "reversed" AI rolling as described above in the youtube video embedded in the question, approximately 19:10 onwards, as the pilot is doing some maneuvering.

So by the way you explain the difference between percentage of this ttwo colour is the direction of the  aircraft travel , meaning more blue on bottom of the sfere then the nose is pointed up

Like the idea of Humpty Dumpty

@GeorgeGeo yes, the way the colors are painted is supposed to signal the direction of flight: blue is up, brown is down. When all you see is brown on you AI, you'd better do something quick :)

What am I missing here?  If the globe of the attitude indicator represents the earth, then when you pitch up the horizon line will go down.  This will show more blue on a conventionally colored instrument, and more brown on the Mig 15.  Unless I am misinterpreting your answer, I think you have it backwards.

It is like an  glass of red wine half empty.

Alright, I see what I missed before.  The Russian attitude display works opposite to what the west is used to, so they inverted the colors to compensate.

@MichaelHall It's not necessarily opposite to western display but opposite to civilian display considering western fighter planes also sometimes use this opposite type of indicator simply due to their robustness

Thanks @ymb1 for the picture, as always, worth a thousand words.

@slebetman, got a recent example to back your assertion?

@MichaelHall Why would it have to be recent for the assertion to be true? Mig-15 is old as well. Very old in fact. More recent Russian attitude indicators were also different from it, with a small symbol of a plane moving on a fixed background https://i.sstatic.net/BoTMI.jpg  The design discussed here is old both for the West and for the East. A Sabre is the same age as Mig-15 https://books.google.cz/books?id=Fdeo9ZVg6dAC&pg=SA1-PA42&lpg=SA1-PA42

@GeorgeGeo Yes, but only if it's the bottom half of the glass that's empty...

@Vladimir, because the assertion used present tense, not past tense.

I had really time hard time wrapping my head around why, with the "simplest possible attitude indicator" described in this answer, pitch indications would be "backwards" from what the pilot would actually see looking at the horizon, while roll indications would not be "reversed".  I could see that it was so, yet there seemed to be a paradox here.  Finally I realized that if a gimballed gyro was used to stabilize a transparent sphere and the horizon line was painted on the back side of the sphere rather than the front side, everything would work "normally".

The situation is not unlike the simplest type of "wet" magnetic compass used in light aircraft.  The compass card swings in the "wrong" direction, opposite to the movement of the actual horizon, as the aircraft turns.  That's because the heading indications are printed on the front side of the disk that comprises the compass card.  Compare and contrast to this type of sailing compass (for example google "LC760 compass") where the heading indications are printed on the back side of the compass card as well as the front side.

If you only pay attention to the back side, the sense of "wrong way" movement is eliminated. I actually once purchased a similar compass to use for some experiments with partial-panel blind flying (turn rate indicator but no other gyro instruments) and found it to be much more intuitive to use than the more common kind.  (And for more food for thought, google "Bohli compass"-- !!)

authors: Vladimir F Героям слава


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Why the windshield wipers move at deferent speed on captain and first officer?
There is that feature linked to the pilot in command at the moment ?

Which airplane?

If they are controlled automatically, then they might vary, although I would expect all wipers to run at the same speed. If they are activated manually and the speed is adjustable, or if each individual wipe is manually triggered, then the speed will depend on who is controlling them, and the captain and first officer might set them to different speeds if control is split between the two pilots.


text:
How is the radius of the danger zone marking for engine inlets calculated?
Does the radius (shown below) take into account smaller objects (FOD), or only people? How is it determined?

Source: Facebook

The ramp/gate area of a typical commercial airport has a rich supply of zipper tags from luggage.  That's probably the biggest FOD risk besides birds.  How to calculate it?  Who knows.

Restrictions on coming to close to the engine area of the Image is the printscreen from Facebook, relation of image to my question is red colored half of this circle in front of the compressor blade. I assume that the max distance is calculated to human not things that are prone to get into the engine.

I assume you do not mean probability? You are asking about how the distance is defined?  This would include calculation or measurement of airflow towards the engine and area & mass of the object, a human for example.

That's got to be a screen shot from a video game of some sort.


text:
What is the location of the CG on the T-38?
Is the Northrop T-38 Talon's CG forward or aft of the center of lift (like the F-16)?

Unlike conventional aircraft (and older fighters), the CG location of the F-16 makes the airplane unstable (making the aircraft more nimble), requiring fly-by-wire and a flight control computer to stabilize the aircraft.
Since the T-38 was designed prior to fly-by-wire technology, I would expect it to have been designed with a CG location that makes the aircraft inherently stable.
If the T-38 does have fly-by-wire, then my answer will get lots of down-votes, but I don't think that is the case looking at this:
https://mitindia.edu/images/pdf/avionics_ppt/stability%20and%20agumentation.pdf

I hate to play grammar police, but double negatives just drive me crazy. They introduce a level of confusion that is just unnecessary. Recommended text:  "I would expect it to have been designed with a CG location that makes the aircraft inherently stable."

The T-38 most certainly was NOT a fly-by-wire aircraft!  Its flight controls are entirely hydraulically powered, but those hydraulics are controlled by the pilot through conventional stick-and-rudders (with essentially a yaw damper to help out with the rudders a bit) -- no fancy computer fly-by-wire used, nor necessary.  Sub-sonic, the plane is nimble but not at all unstable; supersonic, it's very stable.  It's also much older than fly-by-wire technology; in fact it's a good example of how "inherent stability" is what the F-16 could leave behind with FBW and what you gain for doing so.

@Gerry  Done!  Thanks


text:
The F104 tail pipe is not parallel to the runaway when the aircraft taxi.What is the reason?
The tail section is upward for clearance or to vector up the engine exhaust that will help to rise the nose when the aircraft take-off?

Quoting from the Design section of the Wikipedia article on the F-104:

The aft fuselage was elevated from the horizontal reference plane,
resulting a "lifted" tail, and the nose was "drooped". This caused the
aircraft to fly nose up, helping to minimize drag. As a result, the
pitot tube, air inlet scoops, and engine thrust line were all canted
slightly from centerline of the fuselage.


text:
Is the movement of the leading edge flaps not symmetrical on the MiG-29?

Original: russianplanes.net
On this Mig29 in above picture leading edges are at different angles not in line. Same thing happens in flight times when rolling?

I think you are thinking of the "all moving" tail, not leading edges. Most modern fighters do this, not just the MiG-29, but in this picture they are both the same angle...

No, the moving frontal edge of the aircraft  (slats) not stabilators /elevators used for pitching up or down.

At the root of the wings  is clear like daylight just where the leading edges extension meet the wings

The F18 Hornet fly by wire controll this asymmetrical deployment of this part when is in bank and continues to make multiple rolls, but Mig29 is not.

OK I guess I do see some asymmetry there.  On aircraft's right side the sharp "crease" at front of leading edge on fuselage wing root extension, and on actual wing, are aligned, while on the other side they aren't.

Guess the problem is that the pressure in actuators from left side is different from the right side in the stand-by and this creates the difference between the two sides when it is parked on the ground, but in the air that it is not possible due to the functionality of the actuators (high performance need high level of pressure ) .

"in the air that it is not possible" is explicitly contradicted and explained why and how it can happen in John K's answer (the one you accepted). Care to explain why you think it can't?

@FreeMan
Because I believe that there is a system called" asymmetry brake" that does not permit any asymmetrical deflection. Early on  in Bf109 time frame if that happens at slower speed  in banked turn the aircraft spin. Nowadays I getting a sense that is not a problem anymore

No the system in the 109 was not a "driven" system.  Its slats were free floating and could extend asymmetrically during maneuvering.  You are thinking of airliners most of which use power driven slats that only extend with flaps.

Now I read about the take off/ landing schedule  of the slats and flaps 20 /25 degrees for MiG 29. If (the slats here  in discussion ) are free then how in the world can be released only at this anglei if not driven?


text:
The Aim 9 missile mounted on the left side spin in the same direction as right side mounted after launching?
When the AIM 9 sidewinder missile is fired the spin is in the same direction?

Are you asking whether there are two versions of the missile- a left version and a right version?

Yes , but not two versions ,just the fins orientations when they are mounted on left side or right side of the aircraft

@George Geo - No, there is no different fin orientation. Add in a 3rd config, underwing launchers. ex: Station 2 & 8 on an F-16.

Just to add something to @WPNSGuy answer.
A spin is normally imparted to a projectile in order to improve its stability via both an aerodynamic and a gyroscopic effect. Stability here means that the projectile remains more or less fixed on the trajectory given when fired.
AIM-9, like other weapons, works in a different way: it (more or less continually) tracks its target and modify its trajectory consequently. Imparting a spin in order to fix its trajectory would therefore be against its own mission.

Thanks to all for the answers , the question came because I seen an documentary about dogfights in Israel and missile are show that they are spinning in flight after gaining speed

@GeorgeGeo  Some missiles do; the SA-7 (a pretty old missile now, but revolutionary when it was first introduced) has a distinctive corkscrew flightpath, due to the way its seeker works.  More modern missiles, not as much, because that sort of flightpath is aerodynamically less efficient.  Note that depending on how it is produced, a documentary may use CGI "footage" of aircraft and missiles, which is no more realistic than the assumptions programmed into the software used to generate it.

@RalphJ: oh, according to Wikipedia this was due to a limitation in the design of the infrared seeker which had a blind spot just in its middle. As soon as the missile was pointing just straight against its target it became "blind" and had to steer  away to regain sight of the target.

The missile does what it wants to do. No spin is imparted by the missile rail as it leaves, from either side.
Indeed, unlike a bullet, an AIM-9 is not spin stabilized. It does not spin (or tries not to spin). The rollerons on the rear fins counteract any roll.

A rolleron is a type of aileron used for rockets and used to provide
passive stabilization against rotation. While most commonly used to
stablize against roll, it can also be used for counteracting yaw and
pitch as well.

https://en.wikipedia.org/wiki/Rolleron

This is clearly the best answer. It’s beyond me why it was not selected. :-(

@Jim: this is actually the only answer so far, mine is just an addendum, as clearly specified at its beginning. If you are unhappy with GeorgeGeo choice write a comment under its question and discuss the matter with him.


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Is there a procedure for canopy static discharge before the pilot egresses from the cockpit?
Is there a procedure for canopy static discharge before the pilot egresses from the cockpit?

Any static charge acquired in flight is discharged through the tires. Modern aircraft tires conduct electricity well enough for that...

I'm confused about why the word "canopy" is included in this question.  Can you possibly add a little more detail that would help uninformed readers like me understand what the canopy has to do with concerns about static charge?  Is it just because it is made of plexiglass, which is more prone to collecting a static charge than, say, aluminum?

@SaxonXander  https://monroeaerospace.com/blog/why-airplane-tires-are-made-of-conductive-rubber/

@xxavier Huh. Never heard of that in 5 years of flight line maintenance of large aircraft. Always had to connect a ground to the ramp and then the aircraft. We were taught never to touch the aircraft because the static build up in flight would not be dissipated until the ground was connected.

When I was a crew chief on B-52Gs, when we would recover a plane, after wheel chocks were placed, but before the airframe was touched by personnel, a wire was connected to a grounding location in the ramp and connected to a grounding receptacle on the aircraft. Knowing about static bonding, I know that the canopy on smaller aircraft is bonded to the airframe, which means that the static would be dissipated when the grounding wire was attached to the airframe. I have been convinced recently that aircraft tires are made of an electrically conductive material that allows for grounding through them. since this is the case, then the separate ground wire is not for static discharge but rather as a safety measure for refueling. So, the procedure for canopy static discharge is the discharge of static through the tires. The ground crew to grounds the airframe, immediately upon recovery, as a safety measure, but it isn't necessary for static discharge.


text:
Why doesn't the A-4 Skyhawk require rudder in coordinated turns like on other Century Series aircraft?
The Douglas A-4 Skyhawk is a subsonic jet aircraft design from the early fifties.
Usually, in order to make a coordinate turn without skidding or slipping , the  pilots of the other types of aircrafts must move the rudder to the side where they want to turn  alongside with the ailerons and elevators.
However, the A-4 does not require any rudder movements . Use of the rudder is allowed only in cross winds at landing according to a navy pilot writing on a message board:

An every day use would be during takeoff and landing in a crosswind to control the strong tendency for the A4 with the large tail fin to point its nose into the cross wind. This could be disastrous if not brought under control with the large rudder being very effective at slow airspeeds. And of course the spoilers dumping lift on landing so that aircraft weight went on to the main wheels for more effective braking and less tendency to weathercock.
In my day learning to fly with the RAAF - before rejoining the Navy (FAA) - we learnt to fly on Winjeels where all kinds of spins were allowed and practiced. In the Vampire NO. Spins were forbidden - this is where we learnt the 'feet on floor' technique. In the Sea Venom it was a similar situation but by this time we were also flying Vampires in the Navy (1969-70) the technique was ingrained to not use rudders in flight and now not in the Skyhawk.

Is it because of the shape of the wings or the rudder position?

Where’s the question? You seem to have answered your own.

Also, where's your source for the statement that rudder is prohibited?

I can't find the information on the Skyhawk, but it's almost certainly because the airplane has a yaw damper which takes care of minor yaw excursions.  You don't apply rudder for maneuvering if you have a yaw damper, because it messes up the damper's act.

I have a TA-4J NATOPS manual at home I could check later, but I don't remember such a restriction, and agree with John K that it probably wasn't needed due to the yaw damper.  And as GdD said, if you are going to base a question on an assertion you should at least try to substantiate the truth of it.  I didn't read the entire Wikipedia page, did it come from there?

VTC as "Needs details or clarity" - specifically, support for the "is prohibited" statement.

I checked the NATOPS, no mention of using or not using rudder.  If you care to edit the question for clarity I would be happy to provide a better answer for the record.  The question isn't even clear, e.g:  "...pilot had to push the pedal..." and "...does not require any rudder..." contradict each other.  Which one do you think it is, and why?!

https://youtu.be/2eah13pmp9Y in this  you tube interview pilot said that.

@GeorgeGeo I edited the question a bit for clarity, please provide the time on the video when the pilot states the rudder is prohibited item in turns. Just for ease, the interview is very interesting, but more than an hour long

One would see similarities in these procedures and other jet aircraft:  to avoid overstressing the tail at higher airspeeds.  Instead of an aggressive push on the rudder, which could produce a drastic and sudden increase in force on the tail (and hanging ordinance) in a skid, perhaps better to err on the side of slip (a more gentle lateral acceleration), and let the natural shape of the delta wing and tail help it through the turn.

Probably less about overstressing the tail and more about avoiding a nasty abrupt departure from controlled flight.  ("Maneuvering" in this context probably means loaded up at high G in a fight...)  George, help us help you by providing a timestamp.  I don't have the time or interest to listen to the entire podcast, but the context around his remark is important.  While I wasn't a Viet Nam A-4 pilot, my 114 hours in the two seat training version gives me some perspective, and might be just enough to provide a useful answer if I can understand what he is referring to.

Michael Hall, yes "probably less about overstressing the tail and more about avoiding a nasty abrupt departure from controlled flight" In simulation this is happening with rudder deflection.But I don't know why. This was the reason why I asked here.

My opinion is the length of the rudder (not all tail) when deflected.

If you are asking how well your simulator emulates the actual flight characteristics of the A-4, that is a different question.  But in almost any airplane, if you are loaded with Gs and stomp on the rudder things will go poorly.  In case you wonder why this is happening...

Part of me wants to try to answer this, but the question is still a bit messy...  First, pilot technique is not at all the same as a prohibited maneuver.  And second, has it been established that the A-4 is substantially different from other centerline thrust swept wing jets of the era in terms of rudder use?  Most fighters don't need a lot of rudder.  And does the quote match what is said in the video that still doesn't have a time stamp listed for the remark?  It seems that others care more about the question than the OP, and it has morphed...

Based on the included quote, this probably because of the effectiveness of rudder control on the A-4.  If it's effective enough at low speed to control against a high crosswind component for takeoff or landing, it's way too much when at operational speed.
That is to say, if the pilot were to apply proverse rudder in a common turn, he'd push the airplane into a skid by overpowering the applied bank.  The required deflection to coordinate the turn would be so small as to be nearly impossible to apply -- and the A-4 is too old to have computer-managed controls that give more authority at low airspeed and less when fast.  Hence, once clear of the ground, clean, and up to normal operational speed, feet flat on the floor is the order of the day.

Zeis Ikon, but  for small deflection couldn't  pilot use the rudder trim knob?

Not something you'd normally do during a turn (a few seconds to a couple minutes).  Trim is what you'd use, for instance, if under-wing stores on one wing were gone, to counter asymmetric drag.


text:
If the VVI, FPM, airspeed indicator somehow failed you can use fuel flow to help you with your attitude (climbing or descending)?
If the VVI/attitude indicators , FPM, airspeed indicator somehow failed same time and now you could not get cross control check, can use fuel flow counter(because of the decrease consumption rate) to help you with your attitude(climbing or descending) if your aircraft is in the clouds at the moment ?

Well, what would the fuel flow counter read in a spiral dive with the engine screaming?  It may matter whether we are talking about fixed-pitch prop, constant-speed prop, or turbine-- so you might want to specify that in your question-- but the short answer has got to be "no, that the fuel flow counter is not a useful instrument for blind flying."  For fixed-pitch prop, you will get some useful information from the tach and also just from the sound of the engine as another clue to rpms which is related to airspeed.

PS I'm assuming you meant to say that the altimeter has failed too, otherwise it is just too easy as noted in answer below.

Without an Attitude Indicator, changes in altitude at a constant power setting gives you a fairly good indicator of pitch. If your Airspeed Indicator, Vertical Speed Indicator, and Attitude Indicator all failed at the same time, you would use your Directional Gyro and your Altimeter to stay straight and level. You would cross check that with your Magnetic Compass and your Turn Coordinator.
If your Altimeter fails you, you would keep wings level with your DG, TC, and Mag Comp while monitoring your Tachometer RPMs on a fixed prop or your Manifold Pressure on a constant-speed prop. This would give you a more immediate indication of pitch than your fuel flow counter. The DG, TC, and Mag Comp in most aircraft work off of separate systems from each other to avoid all three going inoperative at the same time. They are your instruments to indicate bank and turn.
The ASI, AI, VSI, and Altimeter are all pitot static instruments. And iced over pitot tube and static port can cause them to be inoperable. Using you alternate static port or breaking the glass face of your VSI will make your Altimeter work, making this scenario a non-issue.
Using a built-in GPS or an external AHRS like the Stratus will give you an unofficial altitude in a pinch.


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Is potentially dangerous to select the take-off power on airliner lower that temperature at the moment on the airport?
Is potentially dangerous to select the take-off power on airliner at lower settings  value due to the temperature that could be increased by the time the aircraft will go up?

Are you talking about "flex thrust"?

Yes for Airbus this is the idea

If you're talking about flex thrust, then NO - it is not at all a problem. It is basically a FAKE temperature which the pilots provide the FADEC with so that the FADEC purposely reduces the amount of thrust generated by the engine. This can improve engine lift by reducing wear. But remember, PILOT'S ARE NOT SUPPOSED TO USE FLEX THRUST DURING ROUGH WEATHER. They are supposed to to Firewall their engine (TOGA detent), as they would definetly need max thrust.

No it's not dangerous.  Flex thrust simply exploits all of the runway available, taking advantage of any length that is surplus to the minimum required at max thrust, to allow takeoff with less than maximum thrust to save wear and tear and fuel (wear and tear is a big deal taking off in a sandy/dusty environment and use of flex has a significant effect on engine life).
The fact that you are using "just enough power" for a flex takeoff does not change the technical risks.  The balanced field length requirements (accelerate/stop and accelerate/go safely) are still met, so the extra length available in a non-flex takeoff does not technically speaking, reduce the risks of something going wrong following an engine failure on takeoff or following a reject.

Isn't FLEX setting OAT higher than the actual OAT to obtain reduced thrust?

Yes with flex you set power to give performance equivalent to a higher operating temperature, the "assumed temperature", being the hottest temp at which you can make a departure at max thrust on a given runway for your weight.  You need runway analysis charts for the departure airport to do it. His question is a bit incoherent in its phrasing (language difficulty) and I think he is just asking are there risks to using flex.  Anyway that's the assumption upon which my answer is based.

Yes, that is my question, if there's anything  can go wrong due derating thrust?

@georgegeo if this is your question, you may edit your initial question to reflect it. It is ok to make the description evolve since it adds clarifications


text:
Is there a brightness switch for the AOA gauge on the F-16?
Is there a brightness switch for the AOA gauge on the F-16? Please share a picture with the panel if you can.

I made some edits to hopefully make your question a little more clear. If I got it wrong, please just roll back or edit again!

George's questions seem to attract minus votes because of language barrier. That is just wrong guys, don't be an ass and slap -1 when you don't understand what someone is asking, help 'em out! The main function of this site is to answer questions, not rate them, and definitely not downvote for no good reason. If the Q does't belong here, is totally beyond repair or unclear, vote to close or report it. We rate the A to higlight the best ones and drop the ones that miss the point or are just wrong. Upvoting a Q means you are interested too, but you do not downvote simply because you are not.


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Is there anywhere I can find examples of letters seeking interpretation of FARs, sent to the FAA Office of Chief Counsel?
Is there anywhere I may find examples of letters seeking interpretation of FARs, sent to the FAA Office of Chief Counsel?
What is the appropriate salutation, if one is needed, on a letter to the FAA Office of Chief Counsel?
Should the actual letter (not the envelope) be sent to the "Chief Counsel" or to the "Office of Chief Counsel"?
Should anything in the way letter is addressed, or who it is addressed to, signal that I specifically am seeking clarification of an FAR and not raising some other legal issue?
Related:
How can someone get clarification from the FAA on the intent of a regulation?
https://www.faa.gov/about/office_org/headquarters_offices/agc/key_officials/media/AGC_Org_Chart.pdf

Future edit-- better wording for title "Where can I find"...

The FAA posts the original submission along with the interpretations, at least for recent interpretations. There's a lot of variation in who they're addressed to; many of them are follow-ups to previous contact with the FAA, presumably with the local FSDO.
This one (Standel-West Coast Aviation Services, 2019) might be useful as a starting point, it simply uses the salutation "To Whom It May Concern" and it has the full mailing address in it. It also uses very specific wording about the scope of the request:

This letter is a request for interpretation and clarification of 14
  C.F.R. § 135.267(b).


text:
Does the phrase ".78/280/250" have a standard meaning to an airline pilot, in the context of a long descent?
Does the phrase ".78/280/250" have a standard meaning to an airline pilot, in the context of a long descent?
Specific context-- the table in this answer-- Is the descent performance of the PMDG 737-800 accurate?

OK, appears to be duplicate of https://aviation.stackexchange.com/questions/49028/how-should-i-read-these-climb-and-descent-profile-notations , so I guess I have my answer.

It means the profile is based on descending at 0.78 Mach until the IAS increases to 280 knots, at which point that speed is maintained to 10,000', at which point the aircraft slows to 250 knots for the remainder of the descent until it's time to slow & configure for landing. That 0.78/280/250 is a very standard profile for descent planning in the 737, although variations such as 0.80/320/250 or 0.76/250 are certainly possible.


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Is there any effect that contributes to roll stability even in the absence of sideslip?
Is there any physical dynamic-- involving high wing placement, "pendulum" effect of CG far below wing, or anything else-- that contributes to roll stability in a way that is NOT dependent upon the aerodynamic forces generated by sideslip?
For the purposes of this question, "roll stability" is defined as a tendency to roll towards wings-level, or a reduced tendency to roll toward a steeper bank angle.  Perhaps a better term might be "spiral stability".

I've actually thought of two such effects but will take time formulate a careful answer before posting

Uh oh! The dreaded "pendulum effect." I hope Peter Kampf is not listening!

Paragliders are fascinating because they have tremendous anhedral geometry and yet are fairly roll-stable (much better than hang gliders for cloud flying without adequate instrumentation-- a h.g. pilot is connected to the glider by a single flexible strap so his weight can be viewed as acting at the point where the strap connects to the glider, while a paraglider's lines act essentially like rigid struts and fix the pilot's effective weight far below the wing.) See also free-flight model airplanes w/ parasol config. But, I'm NOT saying here that the "pendulum effect" does not involve sideslip.

I can't find it at the moment, but PK and I had a long comments discussion about the "pendulum effect." I came away convinced that it doesn't exist for a rigid airplane. But, for the reason you mention above, the flexible lines, think it might apply to paragliders, etc. I keep wanting to ask the succinct question of, "does the pendulum effect exist?" But I think it would just get closed as a dupe of one of the questions I linked above.

See my new answer to an old question! -- https://aviation.stackexchange.com/questions/53437/does-pendulum-effect-apply-to-hang-gliders-or-any-aircraft/56548#56548

Some pilots tilt their heads to match the horizon. That shifts the center of gravity sideways, does this count? :D (There are light airplanes that are known to respond to the pilot movements forward or aft in the seat)

Buoyancy is one such effect!  Or more specifically, a buoyant force acting above the aircraft CG.  Think hot-air balloon, dirigible, blimp.
I think I've also thought of one such effect involving conventional winged airplanes but will mull it over some more before answering.

Roll stability, as in an opposing aerodynamic moment caused by a roll attitude of an aeroplane: no there is not. These stabilising roll torques are a result of indirect state variables:

Sideslip angle, excluded for this question.
Differential air velocities over the inner and outer wing in a turn.
Damping forces due to roll velocity.

Yes but the effect is very weak or negligible on a high wing aircraft.  Like a lot of things, the effect is easiest to visualize by taking it to the extreme.  Like with paramotors, which get just about all their roll stability, as well as their ability to bank into turns by skidding, from pendulum effect.

Does the pendulum effect work without involving slipping or skidding? From your own description it does not actually sound so.

I would say not because there has to be an opposing force or point of resistance for the pendulum to work against to create a roll moment.  The other aerodynamic roll stability forces require sideslip to work as well.

Wing flex contributes to better spiral stability as far as my simulations shows. But I guess this comes down to dihedral and sideslip again as well as the "pendulum effect". Just imagine how much harder it becomes to roll an aircraft that flexes the wings like that

As an aerospace engineer, what would be the lift and drag penalty for designing an 25 to 1 aspect ratio biplane from a 50 to 1 glider.  The top wing may provide better roll stability while also improving roll response.  What do the computers say about this?

Imagine "outriggers" that are inverted airfoils, mounted far outboard of each wingtip.  In general, if an aircraft is banked, it will turn, which will mean that the outboard wingtip is moving faster than the inboard wingtip.  In this case the "outrigger" on the outside of the turn will generate more downward lift than the "outrigger" on the inside of the turn, creating a roll torque toward wings-level.
This dynamic seems to play a key roll in explaining why hang gliders and "trikes" tend to experience increased roll stability (or decreased roll instability) in flight at low angles-of-attack (high airspeed), despite the fact that at low angles-of-attack, the dihedral-like "downwind" roll torque contributed by the swept or delta shape of the wing in the presence of sideslip, is much less than at higher angles-of-attack.  By "increased roll stability (or decreased roll instability)" I'm referring to an increased tendency to roll toward wings-level or a decreased tendency to roll toward a steeper bank angle.  This is static stability, not dynamic stability.
Note that these aircraft have lots of "washout" and the wingtips are generating downward lift during flight at low angle-of-attack (high airspeed).  In some cases the lower "flying wires" can observed to be slack.
In some cases the "outrigger" effect described above appears to contribute to a dynamic yaw-roll oscillation in these aircraft. This oscillation may be fundamentally different from the well-known "Dutch Roll" oscillation that swept-wing aircraft are sometimes subject to, typically during flight at high angles-of-attack.  For example the timing of the point of maximum sideslip, in relation to the point of maximum bank angle, may be very different in the hang glider/ trike case and the classic "Dutch Roll" case.
For related content on stability and control in hang gliders and "trikes", but not dealing with the washout / "outrigger" effect, see this answer to the related question Does "pendulum effect" apply to hang gliders or any aircraft?

"key roll" should be "key role"


text:
In the US, is there a way for a pilot to legally "maneuver at will" in actual IMC?
In the US, is there a way for a pilot to legally "maneuver at will" in actual IMC?   Let's say you want to practice a series of reversing 360-degree turns and not have to communicate every change in heading to a controller or follow a controller's step-by-step instructions.
For example, is there such a thing as an IMC "practice area" where only one aircraft at a time is allowed to operate under IFR?
Also, is there a distinction between controlled and uncontrolled airspace in this regard?

Granted, most uncontrolled airspace is rather near the ground, not the ideal place to practice-- but a few spots do exist where the floor of the Class E is higher than 1200' AGL--

Related -- I wasn't aware of the older question when I asked the present one -- https://aviation.stackexchange.com/questions/38455/can-you-file-an-open-ended-ifr-flight-plan

You don't have to have permission to fly in IMC in uncontrolled, class G, airspace. However, you must have an instrument rating and be in an IFR-certified aircraft. In uncontrolled airspace, you may fly into IMC as you like and perform whatever maneuvers you want. It's a very good idea to get flight following in order to receive traffic warnings from ATC if you decide to fly within IMC in uncontrolled, class G, airspace.
You do need to get permission to fly in controlled airspace (all classes except for G) if you are operating under IFR, regardless of actual meteorological conditions.
Note: Classes A-D require permission under both IFR and VFR; classes E and G do not require permission under VFR.
If you want to practice in IMC within controlled airspace, you must explain your mission to ATC and obtain a clearance to operate under IFR. If ATC has the space and the time, they may give you a clearance for a specific area and perhaps a block altitude if you want to climb and descend. There aren't officially designated areas for practice, but if you ask local instructors, they will tell you where they usually go. Most of the time, the preference is to get out of controlled airspace if possible; it's just easier.

I've edited for clarity.

A further small clarification: you need an IFR clearance to operate in any controlled airspace (91.173), which includes class E.

You are not going to get flight following in Class G airspace, since by definition it is uncontrolled—i.e. not under the control of ATC. One of the reasons that there used to be so much Class G in the US was because of the lack of radar and radio coverage. There is very little Class G above 1200' now and it is mostly in the Rockies.

That's not right @JScarry, you can get flight following in class G, it's a Radar Traffic Information Service. ATC will give it to you on a best effort basis.

Yes.  You can requests a local IFR, or quadrant clearance and then receive a clearance for a region, such as:

CLEARED TO FLY (general direction from NAVAID) OF (NAVAID  name  and
  type)  BETWEEN  (specified) COURSES  TO/BEARINGS  FROM/RADIALS
  (NAVAID name when a NDB) WITHIN (number of miles) MILE  RADIUS,
And an example:
  “Cleared to fly east of Allentown VORTAC between the zero four five
  and the one three five radials within four zero mile radius.”

Both are from FAA Order JO 7110.65W 4-1-1

Some ATC facilities such as approaches and towers do not issue local IFR clearances any more... It seems the FAA is cracking down on this. You also cannot have a local IFR clearance if you cross ATC sector boundaries.

Why is that being cracked down on?

In my lexicon a "local IFR clearance" is one in which you will not exit the confines of airspace owned by my facility, and therefore we do not enter you into the NAS. Instead we hand-write a flight progress strip for your flight and issue you a squawk from our MSAW-enabled local code bank. But the clearance is just as restrictive as a NAS-level IFR clearance; any deviation from that clearance requires ATC approval, or a quadrant clearance, just like a "normal" IFR clearance. And @Pugz, not sure what you mean about "cracking down." We can also effect a manual handoff on local-code aircraft...

"Maneuvering at will" within a particular area is called "work". This is particularly common for news/police helicopters, which often need to wander around above some event in class B/C airspace. I've only heard it done as VFR, but I don't see a reason why ATC wouldn't allow it for IFR if there's nobody else around. Be clear about the horizontal and vertical boundaries you want in your request, and let them know when you're done working and want to resume normal control.
You may want to call the relevant ATC unit ahead of time to see when/where will give you the best chance of approval and what exactly to put on your flight plan since this is obviously not a standard request.

You might have better luck working with a controller at a Class D tower near you, or maybe Class C. Find an intersection or something identifiable from an approach plate that's in their airspace, request the maneuvers you want.  They can have you report in as you maneuver, and yet still have control if they need room for some one actually landing, or keep you clear of transiting traffice, etc.

A class D tower controller will be very unlikely to grant this request. Most Class D towers are very limited in the amount of radar rules they can apply, not to mention the airspace they own. But the radar controller (who may or may not rotate in to the tower position from time to time) can certainly assist you.

I have done this while operating under IFR in IMC several times. Just ask for a block airspace and tell them what you want. I've found ATC to be very accommodating.

Block airspace, and area clearances are a tool in the box for instrument instruction in less congested areas, and with sailplanes.  Block altitudes work well with lower performance planes when operating in areas of mountain wave.

@mongo that’s exactly what I use those for - instruction

I have also used that to permit photography through openings in the clouds, where maneuvering was required to setup the subject of the photo shoot.


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Does anyone have a diagram of the HC-130J wing dimensions and fuel tank locations?
Looking for a drawing of the HC-130J that includes fuel tank locations and wing dimension (more than just wingspan)

You are looking for something like this except with dimensions?

If you're looking for the technical data, i recommend you the "service news" material, that can be checked out here: https://www.lockheedmartin.com/content/dam/lockheed-martin/aero/documents/sustainment/csc/service-news/sn-mag-v1-v10/V6N4.pdf.
For an overview, there's a diagram:

(diagram link: https://airwingmedia.com/downloads/lockheed-c130-hercules/).

Thanks, great resources.


text:
What is the current regulatory progress regarding autonomous aircraft carrying passengers?
Are there any lobbying groups working towards this? Has there been any word whatsoever from the FAA on this topic yet?


text:
Easy airplane program
I'm 67 and would like a recommended easy airplane program to play. I cannot afford to really learn to fly so I would like something that is not to complicated to use.  Thanks.

Hi John. Are you looking for specifically a computer game with joystick for control, or would a tablet game be ok?

By airplane program do you mean a flight simulator?

A short(ish) version: for a tabletop computer or a laptop with Windows, any of the Microsoft Flight Simulator series will be fine. They are easy to use, and as you get better, you can add more and more realism.
If you want good graphics, these kind of games typically require quite a powerful computer, but a basic setup will get you started.
If I recall correctly, from MS FS 2004 onwards, you even get other planes flying around, and there is air traffic control you can communicate with via preset phrases.
MS should release a new version in the coming months, but I would remommend an older version, as one can probably find the a ta fraction of the price of the coming version(Amazon.co.uk less than 10£).
You should make sure the version you are about to buy is compatible with your Windows version. MS FS 2004 for example should run fine with Windows 10.
There is also X-Plane, I tried one years and years ago, but I recall it being not as easy to use. Flying I guess was pretty much the same: close enough for the real thing.

I'm an alpha-tester for the upcoming Microsoft FS... you should beware that it is a 83 gigabyte download (currently, this will probably go up), and no, I'm not at all kidding.

Excellent point, older version come on dvd's. Will the 2020 be download only?

As far as I know, yes, and as much as I'm aware an internet connection is required to play it. Downloading 83 GB on my 14Mb DSL connection was a 4 day affair...


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Mushing glide vs forward slip sink rate

Source: Wolfgang Langewiesche - Stick and Rudder; An Explanation of the Art of Flying

I ran across a mushing glide in Wolfgang Langewiesche's - Stick and Rudder; An Explanation of the Art of Flying book and across this topic.
The author says: This gait is sometimes used by a skilled pilot, during an approach to a landing, to steepen the descent and at the same time make sure that the airplane will not pick up excess speed, as it would if the descent were steepened by diving.
I think that the same (or similar) effect can be achieved with forward slip, which, unlike mushing glide, points the nose down, thus giving a pilot a better view during approach while having a higher airspeed as a hedge from the stall. Besides better visibility and higher stall safety margin I assume (but with no scientific proof) that the descent angle with FS might actually be steeper, especially if a pilot slips at Vfe thus creating a large amount of drag.
That being said I wonder which flight condition/maneuver allows steeper approach and if there's any particular benefit of mushing glide to FS? What are your thoughts of this kind of approach?

It really depends on the aircraft. Some have plenty of rudder authority and sideslip very well, others are better at stall onset with still plenty of elevator and aileron authority.
Now first to the term "mushing glide": This happens at very low speed when the airplane flies far at the backside of its envelope. Flow on the inner wing is already partly separated which causes the high drag. The nose is up because of the extreme angle of attack and the stick is pulled back close to its rear stop.
Not every airplane will be pleasant to fly: Some will show abrupt stalling and cannot be held in this mushing glide. Others have less stall margin between inner and outer wing and the pilot risks an inadvertent roll when the outer wing has an airfoil with poor stall characteristics (think 5-digit NACA series) and one side stalls first.
But there are also many airplanes which can be held easily in this attitude and with responsive ailerons and elevator, too. Those are the ones which lend themselves to a mushing glide. I would only fly this after extensive stall testing at altitude, calm winds and when I know exactly how the airplane reacts to control inputs.
On the other hand, if there is plenty of rudder authority to achieve a substantial sideslip angle in a sideslip, I would't think twice and pick this approach. Ending it is possible with little to none altitude loss whereas a mushing glide needs to be ended at enough altitude to pick up some speed for the flare. Visibility is better and the risk of stalling a lot smaller.

"This gait is used...to steepen descent and at the same time...not pick up excess speed"

...the same effect can be achieved with forward slip?

Yes, you can steepen your descent angle at the same speed with a forward slip.
"This gait" is an extreme example of "short field" landing technique, but first:

any maneuvering near stall speed close to the ground is extremely dangerous.  If you have adequate runway to land on, use it.

Gliders present a unique problem when landing, a very large glide ratio.  A 25:1 ratio means you go half a mile to drop 100 feet.  Going faster than Vbg to glide less efficiently is hopeless, as excess speed in ground effect = does not land.
So an expert pilot in a very docile glider with adequate stall warning buffet can push short field technique to the "mushing" limit on a calm day, but even a 5 knot puff of wind could be fatal, and headwind gradients of more than 5 knots near ground level are very common.
So flaps, spoilers, and forward slip solve the problem of angle of descent control at a much safer higher constant speed.  Spoilers in particular help the glider land at a higher airspeed and, if need be, a high drag device (opening clamshells, parachute, friction brakes on wheels) can help bring it to a stop.
But an expert hand on the elevator, and eye on airspeed, is still the best defense against low and slow stalling.

Thanks for the reply. I didn't quite get what do you suggest as a preferred method? I agree that forward slip will increase speed, however I believe that with high speed together with a slip, the drag will increase substantially more, giving a much steeper descent angle during the first part of the maneuver, giving enough time to bleed the speed off in the second part of the maneuver. Overall, I believe a forward sleep is a safer and preferred method. What is your thought? Cheers.

It is, but as Peter says, it depends a lot on the plane.  I found the forward slip extremely useful in the 172 and did it safely at 65 knots (after extensively testing cross controlled stalls at altitude). The issue with gliders is that if too fast they simply  float along.  This is why spoilers (and brakes) enable one to get down and stop fast.  My "preferred method" is plenty of run way.


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What's the relation between TAS and Mach in tropopause?
I understand that TAS decreases with altitude with constant Mach number in troposphere, however I'm not sure what happens in tropospause, as the temperature is constant, however the density is still decreasing. Does that mean the TAS will continue to decrease, just at a slower rate?

The speed of sound depends only on air temperature, not density. So, for a given temperature, a given Mach number will always equate to the same TAS.
In other words, for a given Mach number, TAS remains constant throughout the constant-temperature region above the Tropopause (ie. 37000ft to 65000ft in the International Standard Atmosphere, ISA).